Several considerations must be taken into account when heating and mixing delicate liquid products, including liquids having no solid particles, liquids having solid particles and paste. Such liquid products include food, such as soup and milk, pharmaceutical liquids and cosmetic products. For example, milk must be heated during the pasteurization process, and soup must be heated for cooking. In carrying out this processing of the liquid products, changes in the liquid product should be minimized. For example, changes in the chemical characteristics, nutritional value and taste of the product should be minimized. In order to minimize such changes in the characteristics of the product, the deposition of the product on heated surfaces, called "fouling", should be minimized or totally eliminated.
Accordingly, equipment which is used to heat and mix delicate liquid products should preferably apply heat to the product without any significant irregularity in the heat distribution profile. Avoiding a significant irregularity in the heat distribution profile minimizes fouling, which causes the undesirable changes in the characteristics of the product. In addition, such equipment should include a sealing device to avoid loss of the liquid product or heating medium or contamination of the liquid product by the heating medium. Finally, such equipment should be easy to clean in the likely event that some fouling occurs. Even if fouling does not occur, the equipment should be simple to open in order to inspect the interior flow paths because such inspection is typically required at "reasonable intervals" during the processing of food.
The velocity of the liquid product and the shape of the fluid flow path influence whether fouling will occur. For example, a fluid flow path which creates even, complete mixing during the processing of food will minimize fouling. Also, the velocity of the fluid, which is dependent on the pressure used to drive the fluid, can improve the mixing of the product which would minimize fouling.
In order to insure uniform heating to minimize fouling, one approach is to use scraped surface mixers, which are presently being built as a single unit. These mixers are complex and expensive machines.
Plate and frame heat exchangers are the most widely used processors for heating food, including pasteurizing milk and cooking soup. A plate and frame heat exchanger is made up of a number of thin, pressed metal plates having holes through which conduits pass. A first conduit contains the product being heated and a second conduit contains the heating medium. The conduits have openings along their length selectively positioned such that the two fluids flow in alternating channels between two plates. The plates are ribbed to increase heat transfer. An elastomeric gasket is fitted around the perimeter of the plates, and the plates are clamped together between two end plates.
Plate and frame heat exchangers typically require a large quantity of expensive gaskets to be fitted around the periphery of the plates. Furthermore, significant clamping pressure must be used to compress the plates together in order to avoid leakage of either the product or heating medium through the gaskets to the surrounding atmosphere. In addition, the ribs, or undulations, in the flow channel between two plates can cause an area of uneven heat distribution where products might be trapped. This entrapment could lead to fouling in this area. Furthermore, a typical plate and frame heat exchanger is difficult to open and inspect.
Thus, a need exists for a new type of processor which can heat and mix liquid products, such as milk and soup, without causing any change in the characteristics of the liquid products. For example, such a processor should minimize changes in the chemical characteristics, nutritional valve and taste of the liquid products. Such a new processor should be easy to clean and inspect, should have an adequate sealing system and should minimize the amount of sealant required.